could somone please explain in detail what are stem cells are and thier function thank you.

try this

But how to get over the age gap between present cell and new stem cell?

For a good, comprehensive discussion about stem cells see this:

 

http://www.vincegiuliano.name/Antiagingfirewalls.htm#Declinestemcelldifferentiationtheory

Stem cells are plunipotentiial cells whose fate is not yet decided. they have all the genes activated and can form any particular type of cells. The potential of treatment of various diseases through the use of these stem cells is high because of their ability to form any specific lineage of cells. Thus any lineage of cells that is not functioning properly can be replaced by new cells that will arise from the stem cells.

There is no age concern for stem cells because except a few types, all cells rapidly divide to produce new cells. The stem cells are highly active as far as their division is concerned and every now and then new cells are produced.